This invention relates to an underground information collecting apparatus which has a steel rod with a first end to be inserted into the ground and which is for collecting on the ground a detection signal representative of underground information detected by a sensor mounted in the first end of the rod.
The underground information collecting apparatus of the type is provided, for example, to a boring machine. As well known, the boring machine has a rotary rod made of steel. An end of the rod is inserted into the ground. In order to obtain information of the end of the rod, for example, a temperature in real time, in the boring machine, it is required to provide the end of the rod with a temperature sensor to collect on the ground a signal representative of underground information detected by the temperature sensor.
An example off a conventional underground information collecting apparatus is disclosed in Japanese Unexamined Patent Publication No. 77863/1994. The underground information collecting apparatus comprises a rod made of steel, a sensor such as a temperature sensor embedded in the vicinity of an end of the rod, and a transmitting circuit section and a transmitting coil of a solenoid type received adjacent to the sensor within the rod. The transmitting circuit section modulates a carrier wave by a detection signal representative of underground information detected by the sensor to supply a modulated signal to the transmitting coil. In accordance with the modulated signal, the transmitting coil produces a magnetic signal which is propagated through the rod. The magnetic signal is transmitted through the rod on the ground. In order to receive on the ground the magnetic signal from the rod, a receiving coil of the solenoid type is provided to the rod on the ground side.
The receiving coil has a center axis wound around the rod which serves as a magnetic core so that the center axis is coincident with that of the rod in axial direction. Accordingly, if the magnetic signal propagates through the rod, an induced signal is produced in the receiving coil. The receiving coil is connected to a receiving circuit section. The receiving circuit section receives the induced signal as a receiving signal from the receiving coil and demodulates the receiving signal to reproduce the underground information.
Incidentally, the receiving coil also produces the induced signal in response to an external magnetic signal which is incident from a direction different from that of its center axis. Such an induced signal is a noise signal resulting from stray electromagnetic noises incoming from external apparatuses different from the underground information collecting apparatus and must be cancelled. However, the above-mentioned underground information collecting apparatus is not provided with a function for cancelling the noise signal. On the other hand, the magnetic signal transmitted from the transmitting coil becomes weaker in intensity on the ground, as a signal transmitting distance, namely, the length of the rod becomes longer. Accordingly, a ratio of the receiving signal representative of the underground information to the induced signal due to the stray electromagnetic noises, namely, a signal-to-noise ratio S/N becomes worse, as the length of the rod becomes longer. This is a disadvantage that the signal transmitting distance (depth) becomes shorter (shallower) in the case of large stray electromagnetic noises (environment).